An occupant protection device such as an air bag device uses a data transmission system where a signal level varies within a predetermined range. For instance, as described in JP-A-H10-166993, an air bag device has satellite sensors disposed in several positions of a vehicle. Each of the satellite sensor has a G sensor for detecting as an acceleration an impact applied to the vehicle. The measuring value measured by the G sensor is converted to a digital signal through an A/D converter built in the satellite sensor, and then sent it to an air bag ECU via a transmission interface such as an I/O circuit. The air bag ECU determines presence or absence of the impact to control start of the air bag based on the received measuring value (i.e., impact acceleration).
In the above air bag device, to prevent the air bag from being mistakenly started, air bag expansion is designed to be not executed based on the measuring value of a single G sensor, but executed based on an AND condition with the measuring value of an additional sensor. As the additional sensor, a mechanical safing sensor or another G sensor capable of outputting the measuring signal corresponding to an impact acceleration applied to a vehicle can be used for this purpose.
When multiple G sensors are used for this purpose, the multiple G sensors are disposed in different positions within the vehicle. Based on the measuring values from the differently disposed G sensors, controlling the start of the air bag is executed. Differences among the measuring values from the different G sensors enable analyzing of transmission of the impact acceleration, determining the magnitude or strength level of the impact or the like.
Here, a G sensor in the proximity of an impact occurrence position needs to detect a very high level of an impact acceleration, while a G sensor away from the impact occurrence position needs to detect a relatively low level of the impact acceleration. In this case, a certain G sensor having a broad measuring range capable of detecting the high level of the impact acceleration tends to have a rough (or low) resolution, so that it is difficult for this certain G sensor to accurately measure the low level of the impact acceleration.
To measure the different levels of the impact acceleration, for instance, it is proposed that each of the satellite sensors has two G sensors that have a large range and a small range. When an impact occurrence position is near the satellite sensor, the G sensor having the large range is used for detecting. By contrast, when an impact occurrence position is away from the satellite sensor, the G sensor having the small range is used for detecting.
However, adopting this proposal increases the number of G sensors, and complicates a configuration for transmitting the measuring values from the two G sensors.